Rename Value::CustomValue to Value::Custom (#12309)

# Description
The second `Value` is redundant and will consume five extra bytes on
each transmission of a custom value to/from a plugin.

# User-Facing Changes
This is a breaking change to the plugin protocol.

The [example in the protocol
reference](https://www.nushell.sh/contributor-book/plugin_protocol_reference.html#value)
becomes

```json
{
  "Custom": {
    "val": {
      "type": "PluginCustomValue",
      "name": "database",
      "data": [36, 190, 127, 40, 12, 3, 46, 83],
      "notify_on_drop": true
    },
    "span": {
      "start": 320,
      "end": 340
    }
  }
}
```

instead of 

```json
{
  "CustomValue": {
    ...
  }
}
```


# After Submitting
Update plugin protocol reference

crates/nu-command/src/debug/explain.rs

@@ -293,7 +293,7 @@ pub fn debug_string_without_formatting(value: &Value) -> String {
         Value::CellPath { val, .. } => val.to_string(),
         // If we fail to collapse the custom value, just print <{type_name}> - failure is not
         // that critical here
-        Value::CustomValue { val, .. } => val
+        Value::Custom { val, .. } => val
             .to_base_value(value.span())
             .map(|val| debug_string_without_formatting(&val))
             .unwrap_or_else(|_| format!("<{}>", val.type_name())),